The Class-E family of switching Power Amplifiers (PAs) finds favor in applications requiring efficiency. In a conventional, single-transistor, Class-E PA, ideally 100% efficiency is achieved (in the absence of device and passive component losses) through the achievement of “Class-E switching conditions.” “Class-E switching conditions” refers to the phenomena of Zero Voltage at Switching (ZVS) and Zero Derivative of Voltage at Switching (ZDVS) of the output node voltage. For conventional, single-transistor Class-E PAs, these conditions can be achieved by placing a “Class-E output network” (which can include a parallel combination of an inductor to a supply voltage and a tuned load network) at the output node.
In a particular technology (e.g., CMOS, SiGe, etc.), the output power that can be obtained from a Class-E PA employing a single transistor can be limited by the breakdown voltage of that technology. Thus, a common practice to boost the output power is to stack two or more transistors, thus doubling the breakdown voltage at the final output node. In a stacked configuration, while all stacked devices may be explicitly driven, it is also possible to drive only the bottom-most device and allow the output swing of each device to turn on/off the device above it, thus reducing input power and enhancing efficiency. The main challenge in stacking multiple devices is to retain the Class-E-like behavior for all the stacked devices. Deviation from Class-E behavior leads to degradation in performance in terms of output power and efficiency. Typically, to mitigate this issue (with two stacked devices), an inductor is connected from the intermediate terminal of the stacked devices through a DC-blocking capacitor to ground, or a feed-forward capacitor is connected from the output node of the top device to the intermediate terminal of the stacked devices. However, these techniques are only partially successful in achieving Class-E behavior of both/all devices, even in an ideal situation when there are no losses in the circuit.
Accordingly, new circuits for providing Class-E power amplifiers are desired.